1. Field of the Invention
This invention relates generally to a camera having electronic exposure control apparatus, and more particularly, to improved exposure control apparatus for compensating for the reciprocity failure characteristic of film in the camera.
2. Description Relative to the Prior Art
The speed of a photographic film is an expression of its sensitivity to light. Film speed is measured by subjecting a film to a range of exposures under standard conditions of illumination. The film blackening (density) produced after film processing is measured and plotted (usually logarithmically) against the exposure. The speed of the film can be derived from the resulting graph, which is known as the characteristic curve.
Film speed is defined as a function of the exposure at a specific point of the characteristic curve. One commonly preferred method of determining film speed is defined by the formula: Speed=0.8/E where E is the exposure corresponding to the point on the characteristic curve at which the density is 0.1 above the base plus fog density (density in the negative not forming part of the photographic image).
Film exposure equals illumination (light intensity) multiplied by film exposure time. According to the reciprocity law of photochemistry, so long as the exposure (the product of illumination and exposure time) is held constant, the photographic effect will always be the same, i.e. if intensity is doubled and exposure time halved, the density produced should be unaffected. In practice, a photographic emulsion does not strictly follow this law. A given emulsion is most sensitive to illumination of a particular intensity. When light intensity varies greatly from this ideal, then the reciprocity law no longer applies. Very low light intensities with correspondingly long exposure times, or very high light intensities with very short exposure times, produce less effect. The loss of sensitivity for low intensity--long time-film exposures is known as low intensity film reciprocity failure. The loss of sensitivity for high intensity--short duration-film exposures is known as high intensity reciprocity failure. In either case more than a calculated exposure may be required.
For black-and-white films, the loss of effective speed is relatively unimportant because of wide film exposure latitude. On the other hand, with multilayer color films, which have a more narrow exposure latitude, it is often necessary to give more than a calculated exposure when light varies from a particular intensity. This is particularly true for a photographic process that has a narrow exposure latitude such as utilized in self-processing cameras.
U.S. Pat. No. 3,711,721 discloses camera exposure control apparatus having a photocell that produces a photocurrent, related to light intensity, for charging a timing capacitor. When the charge on the capacitor reaches a predetermined level, a shutter closing signal is produced. Low intensity reciprocity failure of the film employed is intended to be accounted for by progressively attenuating the photocurrent during film exposure. This is done by energizing the photocell with a pulse drive signal, the duty cycle of which is progressively reduced during the exposure interval. By this apparatus, for any given level of scene brightness the photocell produces a photocurrent which is correspondingly progressively reduced. The rate at which the capacitor is charged is reduced as a function of time to produce somewhat longer exposure intervals and, accordingly, to produce exposures, when the film is exposed to low intensity light, that are somewhat larger than a calculated exposure, to account for low intensity reciprocity failure.
This circuitry is relatively complicated. Accordingly, it suffers from a disadvantage in that it would not be readily manufacturable in integrated circuit form by a high yield integrated circuit process. Furthermore, a photocell does not respond instantaneously in an accurate manner to light when it is first turned ON. By sequentially turning the photocell ON and OFF at a high rate during an exposure, the photocurrent may not accurately represent the intensity of the light. In addition, because the photocurrent is attenuated in proportion to the exposure interval, no meaningful compensation can be provided for high intensity reciprocity failure.